1. Field of the Invention
The present invention relates to a focus detecting system and, more particularly, to a focus detecting system for detecting a focusing state of an imaging optical system on the basis of signals, of an image formed by the optical system, generated from three light-receiving sections which are disposed at positions substantially corresponding to points in front of, on and behind a predetermined focal plane of the optical system.
2. Description of the Prior Art
Several focus detecting systems have been proposed in, for example, U.S. Pat. No. 3,532,045 (corresponding German DOLS No. 15 47 457); U.S. Pat. No. 4,277,156 (corresponding German DOLS No. 29 23 573); and Japanese Laid-Open patent applications No. 7336/1976, No. 79425/1978 and No. 106421/1980, which are capable of detecting a focusing state of an imaging optical system on the basis of signals, of an image formed by the optical system, generated from three light-receiving sections disposed at positions substantially corresponding to points in front of, on and behind a predetermined focal plane of the optical system.
When an image of an object is at the boundary of a light-receiving section of a photoelectric transducer element or a predetermined detection range (the field of view) within which substantial signal processing is performed by an electric signal processing system, various transient states are caused when the image enters or leaves the field of view, resulting in unstable focus detection signals. Especially with systems which compare the sharpness of the images for focus detection, errors at the edge of the field of view due to a difference in the sharpness or magnification of the image are big factors for causing errors in focus detection. This is, however, inherent in focus detecting systems of this type. It is desired to eliminate this phenomenon in order that stable focus detection can be performed constantly and without errors.